Rectifier circuits are well known in the art and are frequently used within a power supply circuit which generates a regulated DC output for use by electronic circuit components. The rectifier circuit is typically coupled between the AC source, from which the power supply circuit derives unregulated power, and the voltage regulator portion of the supply. The function of the voltage regulator portion is, among other things, to provide a substantially constant output voltage irrespective of variations in the AC source. The rectifier circuit, however, normally provides a rectified output that follows variations in the AC source to which it is coupled. Thus, the voltage regulator must be capable of performing its function for an anticipated wide range of rectifier output voltages. However, regulators are normally rated for operation with an input voltage that varies over only a limited range. For example, a typical input voltage range for a regulator may be 10 to 50 volts, i.e. a voltage variation of one to five. Thus, if the AC source voltage, and hence the rectifier output voltage, is expected to vary over a range greater than that for which the regulator to be used is rated, application of a single such regulator may not be suitable. One known solution to this problem is to apply multiple regulators to the rectifier output either in cascaded or parallel connection. The multiple regulators represent additional expense, take up additional space and have an adverse effect on overall power supply reliability.
One example of a variable voltage AC source which has an exceptionally wide output voltage range is a 3-phase permanent magnet alternator. Such an alternator includes a mechanically driven shaft carrying a plurality of permanent magnets. One use of such alternators is in aircraft where they are driven by the aircraft engine. Since the alternator generates an AC voltage whose magnitude is in direct proportion to the rotational speed of the mechanical driver, in aircraft applications, no-load voltages can vary over a range exceeding twenty to one.
The electrical output of such alternators in aircraft applications is typically coupled to a switched mode type of power supply, a supply type well known in the art. One characteristic of a switched mode power supply is that it draws substantially constant power from the source to which it is coupled irrespective of the source voltage. As a result, when the alternator output voltage is low, a proportionately higher current will be drawn by the power supply in order to maintain a constant power input. However, due to the large series inductive reactance often found in such alternators, the alternator output voltage will be further depressed by the higher current. Thus, the overall output voltage range of such an alternator when coupled to this type of power supply can exceed the no-load voltage range.
One solution to the problem of accommodating a large alternator output voltage range known in the art is to provide the alternator armature with winding taps. The taps are utilized to shunt portions of the winding as a function of output voltage in order to reduce the overall output voltage range. This solution requires the fabrication of a special alternator with winding taps, as well as control circuitry and a tap changing mechanism for effecting the appropriate shunt connections between taps.
When the alternator output voltage range exceeds the rating of the regulator to be applied thereto, and multiple regulators, as previously described, are applied to a high impedance alternator capable of producing a wide output voltage range, an additional problem results. That is, the components of the regulators must be capable of withstanding both the high currents experienced during low voltage conditions and the additional voltage stresses imposed during high voltage conditions.